1. Field of the Invention
This invention concerns an enhanced oil recovery process and more specifically, a surfactant flooding enhanced recovery process. Still more specifically, this invention is concerned with an oil recovery process usable in subterranean oil formations containing water having abnormally high salinities and/or concentrations of divalent ions such as calcium and magnesium by the use of an aqueous fluid containing a primary anionic surfactant and a solubilizing co-surfactant in a critical, determinable ratio.
2. Description of the Prior Art
Petroleum is normally recovered from subterranean formations in which it has accumulated by penetrating the formation with one or more wells and pumping or permitting the petroleum to flow to the surface through these wells. Recovery of petroleum from formations is possible only if certain conditions exist in the formation. The petroleum must be present in the formation in an adequate concentration, and there must be sufficient permeability or interconnected flow channels within the formation to permit the flow of fluids therethrough if sufficient pressure is applied to the fluids. When the formation has natural energy present in the form of an underlying active water drive, or gas dissolved in the petroleum which can exert pressure to drive the petroleum to the producing well, or a high pressure gas cap above the petroleum saturated portion of the formation, this natural energy may be utilized to recover petroleum. Recovery of petroleum by use of natural energy as described above is referred to as primary recovery. When this natural energy source is depleted, or in those instances where the formation does not contain sufficient natural energy to support primary recovery, some form of supplemental or enhanced recovery process must be applied to the formation in order to extract petroleum therefrom. Supplemental recovery is sometimes referred to in the art as secondary or tertiary recovery, although in fact it may be primary, secondary or tertiary in sequence of employment.
Water flooding, which involves the injection of water into the subterranean, petroliferous formation for the purpose of displacing petroleum toward the producing well, is the most economical and widely practiced supplemental recovery method. Water does not displace petroleum with high efficiency, however, since water and oil are immiscible, and also because the interfacial tension between water and oil is quite high. Persons skilled in the art of oil recovery have recognized this inherent weakness in water flooding and many additives have been described in the literature for decreasing the interfacial tension between the injected water and the formation petroleum in order to increase the oil displacement efficiency. For example, U.S. Pat. No. 2,233,381 (1941) discloses the use of polyglycol ethers as surface active agents or surfactants to increase the capillary displacement efficiency of an aqueous flooding medium. U.S. Pat. No. 3,302,713 discloses the use of petroleum sulfonates prepared from a specific boiling range fraction of the petroleum feed stock for a surfactant in oil recovery operation. Other surfactants which have been proposed for oil recovery operations include alkylpridinium salts, alkyl sulfates, alkylaryl sulfates, ethoxylated alkyl or alkylaryl sulfates, alkyl sulfonates, alkylaryl sulfonates, and quaternary ammonium salts.
The above described surfactants are satisfactory in some formations, particular wherein the salinity as well as concentration or divalent ions in the formation water is relatively low. Generally, the salinity must be less than about 10,000 parts per million and the concentration of divalent ions must be less than about 500 to 1,000 parts per million in order to permit the use of the most commonly available primary anionic surfactants such as petroleum sulfonate.
Persons skilled in the art have recognized the limitation of simple anionic surfactants such as petroleum sulfonate and have described the use of certain solubilizing co-surfactants therewith. U.S. Pat. Nos. 3,792,731; 3,811,504; 3,811,505; and 3,811,507 describe certain mixtures of alkyl or akylaryl sulfonates and nonionic surfactants which exhibit satisfactory performance in petroleum formations having high salinity and/or hard water. U.S. Pat. No. 3,508,612 (1970) describes the use of a dual surfactant system comprising an organic sulfonate such as a petroleum sulfonate and a sulfated, ethoxylated primary or secondary alcohol, which is compatible with high salinity and/or high divalent ion-containing formation water. U.S. Pat. Nos. 3,827,497 and 3,890,239 relate to oil recovery fluids and processes which are compatible with high salinity formation waters and involve organic sulfonate and sulfonated, ethoxylated alcohol mixtures.
While the aforementioned multi-component systems can be rendered soluble in high salinity and/or high divalent ion concentration formation waters, their use has not always been satisfactory because the ratio of the concentrations of the primary anionic surfactant and the solubilizing co-surfactant are extremely critical and difficult to determine and varies with the salinity, divalent ion concentration, as well as with the specific surfactant composition being employed. If too little solubilizing surfactant is used, the primary anionic surfactant precipitates in the presence of the high salinity water. If too much solubilizing surfactant is used, the material is rendered so soluble in water that its effectiveness for purpose of reducing the interfacial tension between the drive water and the formation petroleum is greatly reduced. In either case, oil recovery falls off sharply. Moreover, the cost of the solubilizing co-surfactant is generally two to five times as great as the cost per pound of the primary anionic surfactant, and the use of excessive amounts of solubilizing co-surfactant renders an oil recovery process economically attractive.
U.S. Pat. No. 3,916,997 (1975) describes the use of an oil-external micellar dispersion wherein the concentration of surfactant and alcohol used as a solubilizer are varied to produce a fluid having an electrical conductivity above a specified value.
In view of the foregoing discussion, it can be apprecitated that there is a substantial, unfulfilled commercial need for an efficient and economical petroleum recovery method applicable to formations containing high salinity and/or high divalent ion concentration.